It is known that filtration (micro-, ultra-, nano- or hyperfiltration) membranes are sensitive to clogging by various types of substances: dissolved substances, such as organic materials, substances in the colloidal state, such as metal hydroxides, or, in general, substances in suspension (suspended matter or SM). Clogging results in a very substantial reduction in the filtration capacity of the membrane, the reduction in capacity not always being reversible, the effectiveness of membrane cleaning depending considerably on various factors, such as the effective residence time of the fouling product on the membrane, its relative solubility in the cleaning products and the chemical or physico-chemical interactions between the said fouling product and the surface of the membrane, the latter factor being eminently variable according to the chemical composition of the polymer constituting the membrane.
It is also known (see especially “Mémento Technique de l'Eau [Water Technical Memorandum]”, Volume 1, Chapter 3.1 and Chapter 4.1, published by DEGREMONT, 1989) that coagulation makes it easier to remove the suspended and colloidal matter. In particular, a person skilled in the art knows that coagulation by metal salts makes it possible to destabilize the colloids and to precipitate certain organic materials after adsorption, for example on metal hydroxides. There are several approaches for characterizing this phenomenon:                by a coagulation-flocculation test in a laboratory beaker with various doses of metal salt and estimation, for example, of the settling rates;        by measuring the zeta potential (ZP) and especially the variation in the said ZP as a function of the doses of metal salt added, until that dose which makes the ZP zero, and which therefore corresponds to the required level of treatment for obtaining optimum coagulation, is determined.        
These two approaches lead to a definition of a coagulant dose called the “optimum coagulation dose” which, from the experience gained by those skilled in the art, is the dose which allows the best clarification treatment of the water being treated and which, consequently, will ensure the optimum working conditions of the membrane (that is to say the least fouling conditions).
The drawback of such a treatment involving this optimum coagulation dose is that this dose is relatively high and has repercussions on the operating cost of the clarification treatment and also on the cost of investing the corresponding equipment.
Moreover, it should be noted that most membrane suppliers and suppliers of nanofiltration and reverse osmosis equipment insist, for fear of loss of the guarantees associated with the membranes, on feeding them only with water having a zero, or at the very least a very low, content of heavy metals such as divalent or trivalent ions, in particular such as ferric ions.
Thus, in the literature there are many publications mentioning the use or the injection of one or more metal salts upstream of the membrane treatments. It should be emphasized that these publications mentioned doses close to that making the ZP zero or, at the very least, high doses, close to 30% and more of the said optimum dose for making the said ZP zero.